1. Field of the Invention
The present invention relates to an image pickup apparatus for imaging a document such as a book from above, which is provided with a mechanism to detect curvatures of document surface to perform automatic focusing.
2. Description of the Related Arts
Conventionally, there is known a camera type image pickup apparatus which substitutes an area sensor for a silver salt film. In this apparatus, within one area of an object, a lens is focused only on a main portion of the area. There is also known an apparatus in which the silver salt film is replaced by the line sensor capable of scanning movement (see, for example, Japanese Laid-Open Utility Model Publication No. 50-78114).
However, in an image pickup apparatus using an area sensor, when reading and imaging a document from above, especially in case of a document bound in book form, if a lens is controlled to be in focus on a main portion within the area of the document, other portions can be out of focus, since a paper surface is normally curved in a region adjacent a Joint of the book, wherein a height of document surface is varied. This causes a problem in that an image is blurred and that a clear image is not obtainable. The above Japanese Laid-Open Utility Model Publication No. 50-78114 discloses nothing about a focusing operation. Meanwhile, as shown in United States Patent No.5,084,611, there is disclosed an apparatus capable of expansional processing of a picked up image in accordance with a degree to which a document is curved. However, in connection with this apparatus, there is no disclosure directed to a focusing operation.